1. Field of the Invention
This invention relates generally to plasma ion sources and more particularly to methods for providing RF power, magnetic filtering, and high voltage isolation to an inductively coupled plasma ion source
2. Description of the Prior Art
Focused ion beam (FIB) tools are used for nanometer scale precision material removal. The benefits of FIB tools include nano-scale beam placement accuracy, a combined imaging and patterning system for accurate sample registration and pattern placement, and low structural damage of the area surrounding the removed volume. However, conventional FIB systems typically have a maximum removal rate of ˜5 μm3/s that limits their usefulness for removing volumes with dimensions exceeding 10 μm. Conventional FIB systems are further limited by the low angular intensity of the ion source, so at large beam currents the beam size dramatically increases from spherical aberration. For high beam currents and hence high removal rates of material, a high angular intensity is required, along with high brightness and low energy spread.
Focused ion beams are often referred to as ‘primary’ ion beams when used on secondary ion mass spectrometer (SIMS) systems. Here the term ‘primary’ is used to differentiate it from the secondary ion beam. As with FIB tools, SIMS tools use the focused primary ion beam for precise, sputter removal of atoms and molecules from a material surface. SIMS tools are primarily used to determine the spatial distribution of chemical constituents in the near surface region of a material. Oxygen primary ion beams are beneficial to enhance the yield of positively charged secondary ions from the sample material, and hence enhance the sensitivity of the SIMS measurement. Negatively charged oxygen ion beams, not only enhance the yield of positively charged secondary ions, but also result in minimal sample charge buildup when the sample is a dielectric material. The state-of-the-art SIMS instruments employ duoplasmatron ion sources to produce negative oxygen primary ion beams. Duoplasmatrons have insufficient brightness (˜40 Am−2sr−1V−1) and lifetime to produce the spatial resolution required for many SIMS applications. Duoplasmatrons also produce ion beams with a relatively large axial energy spread (˜15 eV), which is also problematic when endeavoring to produce high spatial resolution focused primary ion beams.
One solution is to use an inductively coupled plasma ion source. Inductively coupled plasma ion sources typically wrap an RF antenna about a plasma chamber. Energy is transferred by inductively coupling power from the antenna into the plasma. A RF power supply with 50 Ohm output impedance utilizes an impedance matching network so that the output of the RF supply can be efficiently coupled to the plasma which has an impedance substantially lower than 50 ohms. To extract negative ions from the ion source, it is typically necessary to use a transverse magnetic field near the source aperture to modify the plasma to allow negative ions to leave the plasma and to separate unwanted electrons that are extracted with the negative ion beam.
Other applications for this type of plasma source include its use as the primary ion source for Ion Scattering Spectroscopy (ISS), focused and projection ion beam lithography, proton therapy, and high energy particle accelerators.
In all cases, a high power density is deposited into the plasma from the antenna in order to create a high density plasma, and the plasma is biased to a potential of a hundred Volts or higher with respect to ground. An ion beam is extracted from a small aperture and is accelerated through a bias voltage to produce a fine beam of energetic ions.
One issue in the design of plasma ion sources is how to create an optimum magnetic field near the extraction aperture while a high voltage bias is applied to the plasma chamber. A second issue is how to provide radio frequency power to the antenna when the effective impedance of the plasma is very low compared to common RF power supply output impedances. This is especially problematic because the effective impedance of the plasma varies with plasma chamber gas pressure, gas composition, and the change of state during plasma ignition. A third problem is how to introduce gas into a reactor chamber that is biased to a high voltage without having high voltage breakdown through the input gas line.
Accordingly, the need arises for new designs and methods that provide efficient RF power to plasma ion sources, as well as improve the magnetic filtering and high voltage stability of plasma ion sources.